Effect of Body Size on Methylmercury Concentrations in Shoreline Spiders: Implications for Their Use as Sentinels.

Shoreline spiders have been proposed as sentinels to monitor aquatic contaminants including methylmercury (MeHg). The present study examined the effect of spider body size on MeHg concentrations in shoreline spiders. We collected 6 taxa of spiders belonging to 4 families (orb-weavers [Araneidae], long-jawed orb weavers [Tetragnathidae: Tetragnatha sp.], jumping spiders [Salticidae], and wolf spiders [Lycosidae: Pardosa sp., Rabidosa sp., and Schizocosa sp.]) from the shorelines of 14 human-made ponds at the Lyndon B. Johnson National Grasslands in north Texas (USA). As a proxy for body size, we measured leg length (tibia + patella) of each spider. Spider taxa differed by 3-fold in mean MeHg concentration, and MeHg concentrations in 4 of 6 spider taxa increased significantly with leg length. The present study is the first to demonstrate that shoreline spider MeHg concentrations increase as a function of spider body size. Because spider size may account for some within-taxa variation in MeHg concentrations, future studies that utilize spiders as sentinels of aquatic contamination by MeHg or other biomagnifying contaminants should take spider size into account. Environ Toxicol Chem 2021;40:1149-1154. © 2020 SETAC.

Relationship Between Methylmercury Contamination and Proportion of Aquatic and Terrestrial Prey in Diets of Shoreline Spiders.

Terrestrial organisms such as shoreline spiders that consume prey from aquatic food webs can be contaminated with methylmercury (MeHg). However, no studies have examined the relationship between MeHg contamination of shoreline spider taxa and the proportion of aquatic and terrestrial prey in their diets. The present study had 2 objectives: 1) determine concentrations of MeHg in 7 taxa of shoreline spiders, and 2) assess the relationship between concentrations of MeHg in spiders and the proportion of aquatic and terrestrial prey in spider diets. We collected shoreline spiders, emergent aquatic insects, and terrestrial insects from in and around 10 experimental ponds. Methylmercury concentrations were greatest in spiders, intermediate in aquatic insects, and lowest in terrestrial insects. The elevated MeHg concentrations in spiders indicate that they were feeding, at least in part, on emergent aquatic insects. However, variability in MeHg concentration observed among spider taxa suggested that the proportion of aquatic and terrestrial prey in spider diets likely varied among taxa. We estimated the proportion of aquatic and terrestrial prey in the diet of each spider taxon from the nitrogen (δ15 N) and carbon (δ13 C) isotope values of spiders and their potential aquatic and terrestrial prey items. The median proportion of aquatic prey in spider diets varied by almost 2-fold, and MeHg concentrations in shoreline spiders were strongly correlated with the proportion of aquatic prey in their diet. In the present study, we demonstrate for the first time that the degree of connectivity to aquatic food webs determines MeHg contamination of shoreline spiders. Environ Toxicol Chem 2019;38:2503-2508. © 2019 SETAC.

Seasonality of dipteran-mediated methylmercury flux from ponds.

Methylmercury (MeHg) is an aquatic contaminant that can be transferred to terrestrial predators by emergent aquatic insects. We assessed the effects of month and pond permanence on dipteran-mediated MeHg flux (calculated as emergent dipteran biomass × dipteran MeHg concentration) in 10 experimental ponds. Emergent dipterans were collected weekly from permanent ponds with bluegill (Lepomis macrochirus; n = 5) and semipermanent ponds without fish (n = 5) over a 7-mo period (February-August, 2015). We detected a significant effect of month on MeHg flux from 6 dipteran taxa and aggregate MeHg flux, with the highest MeHg flux from herbivorous/detritivorous chironomid midges and predatory midges in March; biting midges, phantom midges and herbivorous/detritivorous orthoclad midges in April; and mosquitoes in August. Aggregate dipteran-mediated MeHg flux peaked in April and then declined throughout the remainder of the summer. We did not detect a significant main effect of pond permanence or a significant month × pond permanence interaction effect on MeHg flux for any of the taxa examined in the present study or for aggregate MeHg flux. Given their ubiquity in aquatic systems and their importance in food webs at the land-water interface, dipterans are important taxa that should not be overlooked as a part of the Hg cycle. Environ Toxicol Chem 2018;37:1846-1851. © 2018 SETAC.

Seasonality of odonate-mediated methylmercury flux from permanent and semipermanent ponds and potential risk to red-winged blackbirds (Agelaius phoeniceus).

Methylmercury (MeHg) is an aquatic contaminant that can be transferred to terrestrial predators by emergent aquatic insects such as odonates (damselflies and dragonflies). We assessed the effects of month and pond permanence on odonate-mediated MeHg flux (calculated as emergent odonate biomass × MeHg concentration) in 10 experimental ponds and the potential risk to nestling red-winged blackbirds (Agelaius phoeniceus) posed by consuming MeHg-contaminated odonates. Emergent odonates were collected weekly from permanent ponds with bluegill (Lepomis macrochirus; n = 5) and semipermanent ponds without fish (n = 5) over an 8-mo period (January-August 2015). The MeHg flux from damselflies, aeshnid dragonflies, and libellulid dragonflies began in March and peaked in April, May, and June, respectively, and then declined throughout the rest of the summer. Odonate-mediated MeHg flux from semipermanent ponds without fish was greater than that from permanent ponds with fish. Nesting of red-winged blackbirds overlapped with peak odonate emergence and odonate-mediated MeHg flux. Because their diet can be dominated by damselflies and dragonflies, we tested the hypothesis that MeHg-contaminated odonates may pose a health risk to nestling red-winged blackbirds. Concentrations of MeHg in odonates exceeded wildlife values (the minimum odonate MeHg concentrations causing physiologically significant doses in consumers) for nestlings, suggesting that MeHg-contaminated odonates can pose a health risk to nestling red-winged blackbirds. Environ Toxicol Chem 2017;36:2833-2837. © 2017 SETAC.

Recovery of aquatic insect-mediated methylmercury flux from ponds following drying disturbance.

Small ponds exist across a permanence gradient, and pond permanence is hypothesized to be a primary determinant of insect community structure and insect-mediated methylmercury (MeHg) flux from ponds to the surrounding terrestrial landscape. The present study describes the first experiment examining the recovery of insect-mediated MeHg flux following a drying disturbance that converted permanent ponds with insectivorous fish to semipermanent ponds without fish. Floating emergence traps were used to collect emergent insects for 10 wk in the spring and summer from 5 ponds with fish (permanent) and 5 ponds that were drained to remove fish, dried, and refilled with water (semipermanent). During the 73-d period after semipermanent ponds were refilled, total MeHg flux from semipermanent ponds was not significantly different than total MeHg flux from permanent ponds, indicating that insect-mediated MeHg flux had rapidly recovered in semipermanent ponds following the drying disturbance. Methylmercury fluxes from dragonflies (Odonata: Anisoptera) and phantom midges (Diptera: Chaoboridae) were significantly greater from newly refilled semipermanent ponds than permanent ponds, but the MeHg fluxes from the other 8 emergent insect taxa did not differ between treatments. The present study demonstrates the impact of drying disturbance and the effect of community structure on the cross-system transport of contaminants from aquatic to terrestrial ecosystems. Environ Toxicol Chem 2017;36:1986-1990. © 2017 SETAC.

Effects of fish on emergent insect-mediated flux of methyl mercury across a gradient of contamination.

We examined the effects of fish predation on emergent insect-mediated methyl mercury (MeHg) flux across a gradient of MeHg contamination in experimental ponds. Emergent insects were collected from ponds with (n = 5) and without fish (n = 5) over a six week period using floating emergence traps. We found that the potential for MeHg flux increased with Hg contamination levels of the ponds but that the realized MeHg flux of individual insect taxa was determined by fish presence. Fish acted as size-selective predators and reduced MeHg flux by suppressing emergence of large insect taxa (dragonflies and damselflies) but not small insect taxa (chironomids and microcaddisflies). MeHg flux by small insect taxa was correlated with concentrations of MeHg in terrestrial spiders along the shorelines of the study ponds, demonstrating for the first time the cross-system transport of MeHg by emergent insects to a terrestrial spider.

Relationships between benthic macroinvertebrate community structure and geospatial habitat, in-stream water chemistry, and surfactants in the effluent-dominated Trinity River, Texas, USA.

Over the past 20 years, benthic macroinvertebrate community structure studies have been conducted on the upper Trinity River, Texas, USA, which is dominated by municipal wastewater treatment plant (WWTP) and industrial effluents. The Trinity River is located in the Dallas-Fort Worth metropolitan area, and is the most highly populated and industrialized watershed in Texas. As such, the Trinity River represents a near-worst-case scenario to examine the environmental effects of domestic-municipal and industrial effluents on aquatic life. A 1987 to 1988 study concluded that many stretches of the river supported a diverse benthic community structure; however, a decline in taxa richness occurred immediately downstream of WWTPs. A 2005 study designed to parallel the 1987 to 1988 efforts evaluated how changes in water quality, habitat, and increased urbanization impacted benthic community structure. Physicochemical measurements, habitat quality, geospatial variables, and benthic macroinvertebrates were collected from 10 sites. Surfactants were measured and toxic units (TUs) were calculated for surface water and pore water as indicators of domestic/household use of cleaning products. Total TUs indicated a low potential for biological impacts. Toxic unit distribution was not dependent on WWTP location and did not correlate with any benthic variable. Eight environmental parameters were determined to be useful for predicting changes in benthic macroinvertebrate community structure: surfactant surface water TUs (SWTU), in-stream habitat cover, and surface water total organic carbon were the top three parameters. Abundance, taxa richness, and taxa similarity in 2005 had increased since the earlier study throughout the immediate vicinity of the metropolitan area.

A paleozoological perspective on unionid (Mollusca: Unionidae) zoogeography in the upper Trinity River basin, Texas.

In north central Texas, USA, the zoogeography of unionids in the Trinity River is thought to consist of upland and lowland biogeographic components reflective of differences in upstream and downstream hydraulic conditions. Historical and modern surveys from a limited number of localities were used to delineate these zoogeographic provinces based on the absence of several species thought to occur only in the lower Trinity River drainage. Available zooarchaeological data indicate that at least one species considered absent from the upper Trinity River basin was present during the late Holocene (roughly the last 2500 years), suggesting that both biogeographical provinces shared similar mussel fauna in the recent geological past. The discrepancy between historical and zooarchaeological data is probably the result of inadequate sampling and of an extirpation gradient related to impoundments that have been constructed in this drainage during the last century. The presence of lower Trinity species during the late Holocene in the upper Trinity drainage challenges interpretations drawn from modern biogeographic studies.

A paleozoological perspective on white-tailed deer (Odocoileus virginianus texana) population density and body size in central Texas.

Archaeological and paleontological datasets are used in conservation to add time-depth to ecology. In central Texas, several top carnivores including prehistoric Native American hunters have been extirpated or have had their historic ranges restricted, which has resulted in pest-level white-tailed deer (Odocoileus virginianus texana) populations in some areas. Differences in body size of deer between prehistory and modernity are expected, given that a lack of predation likely has increased intraspecific competition for forage among deer, resulting in smaller body size today. In fact, modern deer from settings without harvest pressure are significantly smaller than those from harvested areas and from prehistoric deer. From a natural history perspective, this research highlights potential evolutionary causes and effects of top-predator removal on deer populations and related components of biological communities in central Texas.

Seasonal dynamics of four potential West Nile vector species in north-central Texas.

A population survey was conducted from April through September 2002 on mosquito species occurring on the Ray Roberts Greenbelt, a riparian corridor used for public recreation on the Elm Fork of the Trinity River, in Denton County, TX. Geographic information system software was used to set up a stratified random sampling design based on habitat parameters. Mosquitoes were collected using light traps, gravid traps, and resting boxes. A total of 29 species was collected during this study belonging to the following genera: Aedes, Anopheles, Coquillettidia, Culex, Mansonia, Ochlerotatus, Orthopodomyia, Psorophora, and Uranotaenia. The four most common species collected during this study were Aedes vexans, Culex erraticus, Culex salinarius, and Psorophora columbiae. West Nile virus (WNV) has been detected in these species in the United States, and they may serve as important vector species in Denton County. Collections were analyzed by the Texas Department of Health for arboviruses. One pool consisting of both Cx. erraticus and Cx. salinarius, collected in August 2002, tested positive for WNV, making it the first mosquito pool to test positive for WNV in Denton County. Canonical correspondence analysis was performed using abundance data of dominant species with selected weather variables and habitat parameters. Important factors for determining dominant species abundance were temperature, precipitation, dew point, and canopy coverage. Spatial and temporal patterns of these species are discussed.